1. Field of the Invention
The present invention relates to digital imaging systems using CMOS photodetectors and more particularly to JFET/CMOS imagers.
2. Description of the Related Art
CMOS Imaging
CMOS (Complementary Metal Oxide Semiconductor) image sensors are specialized integrated circuits (ICs) that act as the “eye” of digital still or video cameras, as well as other electronic equipment. CMOS image sensors detect and convert incident light (photons)—first into electronic charge (electrons) and, ultimately, into digital bits. The sensor core typically consists of an array of photodiodes which detect visible light. CMOS transistors co-located in each picture element (pixel) select, amplify and transfer the photodiode signals. A CMOS imager or imaging system typically includes the sensor core and various ancillary circuits which dynamically amplify the signal depending on lighting conditions, suppress noise, process the detected image and translate the digitized data into an optimum format.
CMOS Developments
Developments in CMOS image sensor technology are paving the way for a new generation of digital imaging products with broad consumer applicability. According to research studies conducted by Intel Corporation, consumers' first preference for a computer peripheral is a digital camera. Digital camera sales are continuing to boom as high quality, full-featured products become affordable for a broad base of consumers. With the ability to provide instantly viewable and easily insertable images into computer-generated documents, the rise in the popularity of the Internet as a communications medium, and most importantly, the elimination of the cost and time of film processing, digital cameras are poised to replace traditional film cameras for many consumer applications.
The total available market for digital imaging, including industrial and security cameras, automotive sensors, PC videocams, scanners, digital still cameras and digital camcorders, is forecasted to grow from about 20 million units in 1996 to over 100 million units in 2002. Most digital imaging devices currently use charge coupled device (CCD) image sensors to electronically capture images. A CCD device is a charge-transfer device that collects photocharge in pixels and uses clock pulses to shift the charge along a chain of pixels to a charge-sensitive amplifier. CCDs typically output pixel-by-pixel analog signals. CCDs have been used for about 25 years to provide image sensing in products including video cameras, security cameras and now digital still cameras as well as fax machines and copiers.
CCD technology has been incrementally engineered to provide high quality images with low visible noise (granular distortion visible in the image). However, CCDs are complex and expensive to use in cameras; they usually require three to eight supporting chips (depending on camera features) as well as multiple voltage sources. In addition, CCDs typically begin to exhibit unacceptably high noise levels above a threshold of roughly 1024×768 resolution (the typical resolution of high-end camcorders) at video frame rates (30 frames per second). The high-resolution CCDs used for high-end applications have been cost-prohibitive for consumer products and have been operated at slow rates to minimize their read noise.
CMOS imager technology, on the other hand, can deliver significant system cost savings and can mitigate the performance limitations of the CCD. A fully integrated CMOS solution can be five times lower in cost and ten times lower in power consumption (which leads to longer battery life, better form factors and reduced weight) than a comparable CCD imaging solution.
Applications
CMOS video imagers can be implemented in widely-used consumer products such as camcorders and digital cameras to create images usable by PCs, as well as in toys, security cameras and digital cellular/PCS telephone systems. Business and industrial applications include videoconferencing, machine vision, medical instrumentation, broadcasting and video-based information display or acquisition for real estate, insurance and other business segments.
CMOS still imagers can reduce the cost and power required to provide high quality image sensing for consumer products including still digital cameras, toys, portable communications devices and security equipment. Business and industrial applications include professional photography for publishing, office automation, machine vision and medical instrumentation.
History of CMOS Imaging
MOS fabrication technology has been used since about 1977 to produce imagers. Early camcorders used NMOS(N-channel Metal Oxide Semiconductor) image sensors which were quite competitive with the nascent CCD-based products of the early 1980's. CCDs proved superior for most imagers, however, because of the immaturity of MOS transistor integration. Specialized imagers subsequently migrated to CMOS in about 1984 to facilitate robust operation, radiation hardness and higher performance for space exploration, aerospace and low-volume commercial applications. Ever shrinking circuit geometries coupled with advances in imager design have now enabled the circuit miniaturization needed to produce CMOS imagers at the higher pixel resolutions needed for compact consumer electronics.
CMOS imagers can offer several significant advantages over CCD imagers, including lower overall cost for the imaging system bill of materials (BOM), lower power requirements and a higher level of integration, enabling “camera-on-a-chip” capabilities that can also reduce the size of the imaging system. In addition, CMOS imagers are usually relatively easy to manufacture in a standard CMOS wafer fabrication facility, whereas CCDs can be complex, expensive to produce, and can require a specialized fabrication facility dedicated to CCDs.
With process geometries of 0.5 μm and below, CMOS imagers can support resolution comparable to that of CCDs. In addition, a single CMOS imager includes anti-blooming and on-chip signal conditioning so objectionable artifacts and noise can be suppressed to very low levels. Blooming is the overflow of signal charge from extremely bright pixels to adjoining pixels that results from oversaturation. Blooming is similar to overexposure in film photography, except that it is manifested in electronic imaging media as vertical or horizontal streaks. Standard CCD cameras, for example, often portray night-time images of cars with two vertical streaks caused by blooming from headlight saturation. Adding saturation-limiting potential barriers and a charge sink adjacent to the pixel can help eliminate blooming. Blooming is not a problem in typical CMOS active pixel sensors because no charge is transferred either within the pixel or pixel-to-pixel.
As process geometries continue to shrink, integration levels continue to increase. CMOS has been positioning itself as the technology of choice for all imaging products. According to industry analysts, CMOS image sensors are expected to gradually replace CCDs in many digital imaging applications and account for nearly 50 percent of digital imagers over the next five years.
Ease of Manufacture
CMOS is used to manufacture approximately 90 percent of all semiconductors today, including most memory devices, microprocessors and application specific integrated circuits (ASICs). Manufacturing imagers using this highly standardized process is typically less costly than producing CCDs, which are usually made using specialized fabrication processes that require dedicated manufacturing equipment. In addition, unlike CMOS, CCDs have not integrated digital logic on the same chip nor supported the level of integration and availability of functional cells.
As a further benefit of the standardized CMOS technology, most semiconductor technology advances occur in CMOS technology, financed by the ongoing efforts of major manufacturers to produce larger memory chips and faster microprocessors. Other devices produced in CMOS will benefit from the ongoing research and investment in new equipment that is required to move to smaller process geometries, which allow more functions to be integrated onto a single chip.
“Camera-on-a Chip” Capability
With CCD imagers, only the image capture and foliation functions have been included on the CCD image sensing device. A handful of supporting semiconductors are usually required to operate the device, condition the image signal, perform post-processing, and generate standard video output. In contrast, CMOS imagers can be made with a “camera-on-a-chip” capability. The fully integrated product can offer five times lower cost and ten times lower power consumption (which leads to longer battery life for portable devices) than a comparable CCD imaging board.
Performance
Resolution and noise are two interrelated parameters that are used to measure performance of a digital image sensor. There are two dimensions of noise: read noise and fixed pattern noise (FPN). Read noise (also known as temporal noise) occurs randomly from time to time and is generated by several basic noise mechanisms of electronic components and looks to observers like the “snow” present on inactive TV channels. FPN, on the other hand, does not change from frame to frame and is somewhat analogous to peering at scenes through a chain-link fence. Depending on its relative magnitude, FPN can be quite objectionable relative to the temporal noise.
CMOS imager technology can readily support future digital imaging devices. Active pixel sensor imagers can offer extremely low read noise at video frame rates because the amplifier at each pixel typically operates at much lower bandwidth than the output amplifier. This basic change can reduce the read noise by at least a factor of 10.
While pixel-based amplification is a key attribute, the fact that each pixel's photo-generated charge is independently converted to a voltage can also be a drawback. Since the amplifiers are not exactly alike from pixel to pixel, the resulting FPN often can limit imaging performance more than temporal noise. A key to successful CMOS imagers has been to integrate the necessary on-chip circuitry to eliminate the fixed pattern noise. Although CMOS readily facilitates such integration, the size of such circuits has presented problems.
Inside a CMOS Imager
CMOS pixel arrays form the core of a CMOS image sensor. CMOS pixel arrays are based on either active or passive pixels. A pixel is an individual picture element. In solid-state imagers, a pixel refers to a discrete photosensitive cell that can collect and hold a photocharge. Photocharge is a phenomenon in which silicon exposed to photons results in the release of charge carriers. A photocurrent results when an electric field sweeps the carriers away. The current that the light generates is directly proportional to the light intensity. A photocharge results when a capacitor collects the charge that the photocurrent carries. The display resulting from the collection of photocharge usually has the same number of pixels as the imager does. Passive pixels typically use a simpler internal structure, which does not amplify the photodiode's signal within each pixel. Active pixels typically include amplification circuitry in each pixel.
The simplest CMOS imaging pixel is a passive pixel, which consists of a photodiode and an access transistor; the photo-generated charge is passively transferred from each pixel to downstream circuits. The charge must, however, be efficiently transferred with low noise and nonuniformity. A Passive Pixel Sensor (PPS)—a CMOS image sensor made with passive pixels—thus shifts the signal processing burden from each pixel to support circuits which read each pixel, convert the charge to a signal voltage, amplify the resulting signal, suppress the temporal noise and cancel the support nonuniformity created by these circuits.
The fraction of real estate within each pixel which detects light is the optical fill factor. A fill factor is the ratio of light-sensitive area to the pixel's total size, also known as aperture efficiency. The fill factor is not 100% because some of the pixel area must be used to transfer the signal to the rest of the imager.
Passive pixel sensors typically have a high fill factor—typically 70 to 80 percent—because the pixel often contains as few as one access transistor. PPS technology has been used for applications requiring a lower-cost image sensor solution, such as transmission of video over standard telephone lines for personal communications and business videoconferencing. The large optical fill factor can maximize signal collection and, hence, minimize fabrication cost by avoiding the need for microlenses. Microlenses—lenses etched directly on the chip's surface for each pixel—are a standard feature of CCDs and many CMOS active pixel sensors. When accurately deposited over each pixel, microlenses concentrate the incoming light into the photo-sensitive region, increasing the effective fill factor. When the as-drawn fill factor is low and microlenses are not used, the light falling elsewhere is either lost or, in some cases, creates artifacts in the images by generating electrical currents in the circuitry.
Active pixel sensors (APS)—CMOS imagers made with active pixels—typically incorporate transistors in each pixel to convert the photo-generated charge to a voltage, amplify the resulting signal and reduce noise. Adding these components typically reduces APS fill factor in 0.6 μm processes to about 20 to 30 percent. To counter the lower fill factor, APS imagers often use microlenses to capture the light that would otherwise strike the pixel's insensitive area. Microlenses can double or triple the effective fill factor. The lower temporal noise of APS imagers make APS imagers very useful for digital still camera applications.
Many other problems and disadvantages of the prior art will become apparent to one skilled in the art after comparing such prior art with the techniques as described herein.